This invention relates in general to the detection of microorganisms and more particularly to a process and machine reading cards into which specimens suspected of containing microorganisms are introduced.
A procedure which has recently been developed for detecting the presence of microorganisms in specimens, and this procedure utilizes cards or cuvettes having wells filled with dehydrated culture media of a highly selective nature. Clinical specimens are diluted in saline solution and the dilutions so formed are introduced into the cards where they mix with and rehydrate the selective media in the wells. Each medium is selective in that it is specific to a particular microorganism, and when that microorganism metabolizes in the rehydrated medium, the medium undergoes an optical change. This change is usually characterized by an increase in turbidity or an alteration of color, and is detected by projecting light through the well containing the rehydrated medium and measuring the intensity of the light beyond the well. By mixing antibiotics with the media, it is possible to determine the susceptibility of an identified microorganism to various antibiotics, for if any antibiotic is effective, the culture medium in which it is present will retain substantially its original optical characteristics.
The foregoing procedure in an elementary form is disclosed in U.S. patent application Ser. No. 461,249 of C. Aldridge et al, filed Apr. 16, 1974, now U.S. Pat. No. 3,963,355, which further discloses several selective media which are suitable for use in this invention. U.S. patent application Ser. No. 528,840 of S. Gibson, et al, filed Dec. 2, 1974, discloses a card suitable for conducting antibiotic susceptibility tests. The U.S. patent application Ser. No. 682,664 of Ronald A. Charles, et al, filed May 3, 1976, and entitled AUTOMATED ANTIMICROBIAL ANALYZER discloses a machine for examining the wells of the cards on a high speed automated base.
Each card, in addition to its wells containing the selective media, has indicia marked upon it for identification purposes. This indicia, which for the most part is in the form of Arabic numerals, may be machine read by projecting light through the card. The interruption of any beam of light indicates the presence of a marking. Particles of foreign matter sometimes accumulate on the card at the identification segments, and these particles will block some light and consequently may provide false readings. Stray marking may also result in false readings.
The wells of the cards and the filling channels leading to them are isolated from the surrounding atmosphere. Each card is loaded with the diluted specimen by evacuating air from its interior and then replacing the evacuated air with the diluted specimen. A small volume of air usually remains in the card and this entrapped air may accumulate as a bubble in one or more of the wells. Bubbles, however, appear opaque to the automated reading apparatus and hence may indicate metabolic activity where none is present. Also, some microorganisms produce gas as they metabolize, and this gas will result a bubble which renders at deceptive reading as to the light transmitting characteristics of the rehydrated medium.
Also, it is not uncommon for the tape which covers the ends of the wells to bulge outwardly into a convex configuration as the card is incubated at elevated temperatures, and this alters the optical properties of the column through which the light is projected, thereby providing another basis for error. This bulging is in part attributable to the expansion resulting from the increase in temperature and in part to the natural production of gases resulting from the metabolic activity of some microorganisms. In any event, the bulging, or lensing as it is called, takes place over a period of about two to three hours and thereafter the distorted tape remains in about the same configuration. Hence a reading taken at the beginning of the period cannot be accurately compared with a subsequent reading.